In the simplest form of organic light emitting diodes (OLEDs) the active organic semiconductor is sandwiched between two metallic electrodes [1, 2]. Since such electrodes are usually opaque in the visible spectral range unless very thin, this type of geometry prevents efficient surface emission. This difficulty can be finessed by using a semitransparent electrode such as indium tin oxide (ITO) as the hole-injecting anode. However for the electron-injecting cathode opaque metallic electrodes such as aluminum (Al) are still been used [3]. In this OLED configuration, due to the relatively high refractive index of the organic active layer, a considerable fraction of the emitted radiation remains trapped in the device as waveguide modes that eventually couple to surface plasmons (SP) excitations on the anode surface, which consequently decay nonradiatively [4]. If nothing is done to recover this trapped waveguided light, then the device efficiency remains always low [5].
On a smooth metal-dielectric interface, light cannot efficiently couple to the SP excitations, which are the elementary excitations of the metal surface, because conservation of energy, E and momentum, k are not obeyed [6]. On an (E, k) plot the SP dispersion curve lies below that of the electro-magnetic waves in vacuum [6]. But in a metal film that is perforated with a 2D periodic array of holes, the periodicity allows grating coupling of the SP to light [7]; this coupling results in surface plasmon polariton (SPP) excitations. The lattice periodicity promotes zone folding of the SP dispersion relation, which results in the formation of SP band structure that makes it possible for light to directly couple to SP excitations. Indeed it was recently found [7-18] that the optical transmission through subwavelength hole arrays fabricated on optically thick metallic films is enhanced at resonance wavelengths (or maxima), where light couples to the film's SPP excitations. If these maxima overlap with the photoluminescence (PL) band of the active organic layer of an OLED, then it might be possible to extract more EL light out of the device [5, 19-21] without compromising the current injection capability of the patterned electrode.